Training Wheels for Electrica Wave Files

7/31/2019 Training Wheels for Electrica Wave Files

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EVALUATING WAVE FILES TO DETERMINE THE SEVERITY OF

THE ULTRASONIC EMISSIONS

We are often asked at which voltages and on what type of equipment isultrasound most effective. The answer is not simple since it often dependson the individual asking the question. First of all, determining the definitionof low, mid and high voltages is relative. Those in the power distribution

arena will look at 12 kV as low voltage, those who work in a typicalmanufacturing plant will look on 440 Volts as high voltage. The mainelectric problems detected ultrasonically are partial discharge, arcing,tracking and corona. Ultrasound detects the ionization of air, as itproduces turbulence. This can be from Corona, Tracking or Arcing. Heatis produced when there is either Resistance due to corrosion or fromTracking and Arcing. Since Heat is produced by the flow of currentthrough corrosion or resistance, you can have a hot spot detectable with

IR, and have no Ultrasound because there would be no ionization.

Low voltages


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Low voltages

voltage inspection will be to combine infrared imaging with an ultrasonicprobe. Please keep in mind that since air cannot be a conductor ofelectricity below 1000 volts, corona cannot exist. Any buzzing soundsare either loose components vibrating at 60 (or 50) Hz or tracking.

Mid and High Voltage inspection

Higher voltages often produce more potential for equipment outage.

Problems such as arcing, destructive corona or tracking (sometimesreferred to as "baby arcing") as well as partial discharges and mechanicallooseness all produce detectable ultrasound that warn of impendingfailure. Detecting these emissions is relatively easy with ultrasound. Theacoustic difference among these potentially destructive events is thesound pattern. Arcing produces erratic bursts, with sudden starts andstops of energy, while corona is a steady "buzzing" sound. Destructivecorona has a build up and drop-off of energy resulting in a buzzing sound

accompanied by subtle popping noises.

If the subject equipment is at a distance while scanning for theseemissions, use a parabolic reflector such as the UE UWC (Ultrasonic WaveForm concentrator) or the UE LRM (Long Range Module). Theseaccessories more than double the detection distance of the standardscanning modules.

Partial discharge (PD) which occurs inside electrical components such asin transformers and insulated buss bars, is another problem that can bedetected with ultrasound. Partial discharge can be quite destructive. It iseffected by and causes deterioration of insulation This is heard as a


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Since all Ultraprobe instruments heterodyne ultrasound down into theaudible range, either the headphone jack or the Instrument itself (theUltraprobe 10000) may be used to record sounds. You must use a suitablerecording device that has a suitable bandwidth in the lower frequencies.

DIGITAL VOICE RECORDERS ARE NOT ACCEPTABLE as they only canrecord signals above 300 Hz, which is not low enough to be useful for the50 or 60 Hz peaks. Laptop Computers, MP3 Recorders or a QualityCassette Recorders work well for recording the signals in the field.

When recording the sound emissions, you need to make certain that thesignal is not distorted. On the Analogue instruments, you should not letthe signal go over 50% of full scale on the signal strength indicator. On theDigital Instruments, you should try to maintain the signal strength tobetween 4 to 6 segments.

These recorded sounds can then be downloaded to a PC with a sound cardand viewed as a spectrum or time series for analysis. It is necessary toexamine both the Spectrum and the Time Domain images to make a finaldecision on the severity.

The main harmonic of an electrical emission (60 Hz in the USA and 50 Hzelsewhere) will be most prevalent in corona. As the condition becomes

more severe, there will be fewer and fewer 60 Hz harmonics observed. Asan example, arcing has very few 60-cycle components. Mechanicallooseness will be rich in 60 Hz Harmonics and will have little frequencycontent between the 60 Hz peaks and will also demonstrate harmonics


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Electrical discharges take place in the Positive portion of the sine wave.Depending on the severity, the discharges take place in different points ofthe sine wave and have different durations in time. For example, for air tobe a conductor of the discharge, you must have over a 1000 volt potential.This means that the actual discharge takes place only at the peak of thepositive portion of the sine wave.

EXAMPLE OF DISCHAGE POINTS FOR CORONA


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TRACKING

When you have tracking, there is a low current pathway to ground acrossan insulator so the discharge does not have to take place at the peak of thewaveform. It therefore can happen anywhere on the positive portion of thecycle. This can be seen by the location of the vertical lines indicated by thearrows on the left side of the diagram. The spacing for the peaks in theTime Series would be similar non uniform spacing. They correspond tothe popping sounds heard in the headphones (Heterodyned Ultrasonicsignal) that are not uniformly spaced in time. As the Tracking becomesmore severe, there are more discharge events and therefore more nonuniform spaced narrow peaks.

ARCING

When you have Arcing, the discharge to ground across an insulator is a

high current path over an insulator that does not require a high potential.This is a burst of energy that lasts for extended periods of time. It isindicated by the arrow on the right side of the diagram. The buzzingSound that is heard with corona and the initial stages of tracking is notheard with arcing. Only the bursts of discharge can be heard. These willbe seen as wide peaks in the time series.

TRACKING


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CORONA SPECTRUM

NOTE THE FREQUENCY CONTENT BETWEEN THE

PEAKS THAT IS ALMOST HALF THE AMPLITUDE &THE RICH HARMONICS OF THE 60 Hz SIGNAL


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CORONA TIME SERIES

Note the uniformity of the peaks inboth spacing and in the amplitude


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LOOSE COMPONENT SPECTRUM

NOTE Lack of FrequencyContent between the peaks


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LOOSE COMPONENT TIME SERIES

NOTE Uniformity in Amplitude of Spikes


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TRACKING SPECTRUM

Note the loss of Harmonics


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TRACKING TIME SERIES

ote non uniform spacing

between discharges


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BAD TRACKING

Note greater number of discharges


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ARC SPECTRUM

Note loss of Harmonics


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ARC TIME SERIES

Note the long duration ischarges with

little or no spacing

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Training Wheels for Electrica Wave Files